The present invention relates to switches and protective devices, and more specifically pertains to combination Positive Temperature Coefficient Resistor (PTC) and Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) devices providing switching and overload protection service.
Fuses are typically used along with switches, relays, or MOSFETs to provide overload protection and switching services, respectively, for different automotive loads. The switches, relays, or MOSFETs selectively connect and disconnect a power supply to a load. The fuse is provided between the power supply and the load in order to protect the load and, in many cases the switching device, from excessive currents. For example, if an electrical short causes excessively high current to be drawn from the power supply, heat (i.e., I.sup.2 R losses) generated by the fuse causes the fuse to bum up (i.e., open circuit). Assuming the fuse is appropriately selected, the fuse open circuits in time to protect the downstream load and the switching device from overload current. A problem with this approach is that burnt out fuses must be replaced which can become costly over the long run as more fuses burn out.
To address the fuse burn out problem, the industry has employed Positive Temperature Coefficient Resistors (PTCs) in place of fuses. (See U.S. Pat. Nos. 4,717,996; 5,619,076; and 5,757,141). PTCs are well known functional analogues of circuit breakers. In general, PTC resistance increases dramatically as a PTC threshold temperature is exceeded. The increased resistance resembles an open circuit, thereby cutting off current and protecting the PTC, a series switch and downstream components. Having a PTC connected in series with a MOSFET in the same electrical system is known (see U.S. Pat. Nos. 4,717,996; 5,619,076; and 5,757,141).
Unfortunately, there are many instances wherein a PTC will not be able to protect a series switching device from thermal damage. For example, if an input control signal to a MOSFET's gate has an excessive delay in transitioning from low to a high, turn-on voltage (i.e., its rising time is too long), the delay slows the MOSFET conduction and the MOSFET heats up. Second, if the MOSFET is to conduct a relatively high current, but the gate's input control voltage is too low to fully turn on the MOSFET, the MOSFET will heat up. Third, a MOSFET heats up when it operates continuously over long duration at abnormally high ambient temperature.
Regardless of the cause of heat generation in a MOSFET, limitation of MOSFET temperature is necessary to avoid device destruction. One approach to minimize MOSFET heating is to provide a heat sink. Another well known technique involves using circuitry to monitor MOSFET operation and modify MOSFET operation when a threshold temperature is approached. Significant problems exist under both of these approaches regardless of whether a PTC or a fuse is used with the MOSFET to provide the overload function. For the purpose of simplifying this discussion, and not to limit the scope of the present invention, unless specified otherwise hereinafter, the remainder of this specification will be presented in the context of a MOSFET/PTC configuration.
With respect to the heat sink approach, three separate devices are required: 1) a PTC; 2) a MOSFET; and 3) a heat sink. In addition to being costly, many heat sinks are relatively large devices and therefore require additional system space. Thus, if possible, it would be desirable to eliminate or at least substantially reduce the size of any required heat sink.
Another problem associated with the heat sink technique rests in the fact that the PTC and MOSFET are located apart from one another. For example, assuming that the MOSFET and an associated heat sink are located in a region of high ambient temperature, the elevated temperature may inhibit effective heat transfer away from the MOSFET. In this case, PTC tripping is the only hope for protecting the MOSFET from overheating.
Three factors affect a PTC's thermal trip point including the PTC's threshold temperature and variance, the ambient temperature of the PTC's environment, and the PTC's I.sup.2 R heat generation. The PTC's threshold temperature and variance are fixed once the PTC is selected for a system, so the impact of the threshold temperature and variance on the trip point is also fixed. In addition, since the PTC is apart from the MOSFET, it may well be in a region of lower ambient temperature than the MOSFET. Assuming a PTC is in a lower temperature environment than the MOSFET, ambient temperature has less impact on PTC temperature than would otherwise be the case if the PTC and MOSFET were located in the same region of high ambient temperature. Moreover, current from the MOSFET to the PTC may be insufficient to generate enough I.sup.2 R heat, in combination with the PTC's slight ambient temperature heat build up, to trip the PTC and protect the MOSFET. Thus, it would be advantageous to have: 1) a PTC located in close proximity to the MOSFET, and therefore, in the same ambient temperature region as the MOSFET; and 2) to provide heat input to the PTC supplementing its ambient and I.sup.2 R heat inputs in order to ensure that it trips in time to protect the MOSFET.
The monitoring circuitry approach to limiting MOSFET heating also has shortcomings. The monitoring circuitry requires use of system space in addition to that required for the PTC and MOSFET. Also, the monitoring circuitry increases the cost associated with an overall system. Additionally, if the monitoring circuitry is unable to adequately control the MOSFET's heat build up, then reduction of current through the MOSFET caused by tripping the PTC may be the only way to protect the MOSFET. However, since the PTC and MOSFET are located apart from one another, the problem of having the MOSFET in a region of higher ambient temperature than that for the PTC still exists. In other words, the PTC may be in a region of relatively low ambient temperature, and it therefore may not be able to reach its thermal trip point before MOSFET damage
occurs. Also, the PTC still has only two heat inputs to help it reach its thermal trip point in order to protect an overheating MOSFET.
Thus, there existed a need for a simple, inexpensive switching configuration which protects against excessive current and also protects an associated switch from thermal destruction.